17,933 research outputs found
Two physical characteristics of numerical apparent horizons
This article translates some recent results on quasilocal horizons into the
language of general relativity so as to make them more useful to
numerical relativists. In particular quantities are described which
characterize how quickly an apparent horizon is evolving and how close it is to
either equilibrium or extremality.Comment: 6 pages, 2 figures, conference proceedings loosely based on talk
given at Theory Canada III (Edmonton, Alberta, 2007). V2: Minor changes in
response to referees comments to improve clarity and fix typos. One reference
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Quasilocal energy and naked black holes
We extend the Brown and York notion of quasilocal energy to include coupled
electromagnetic and dilaton fields and also allow for spatial boundaries that
are not orthogonal to the foliation of the spacetime. We investigate how the
quasilocal quantities measured by sets of observers transform with respect to
boosts. As a natural application of this work we investigate the naked black
holes of Horowitz and Ross calculating the quasilocal energies measured by
static versus infalling observers.Comment: 5 pages, 1 figure; submitted to the 8th Canadian Conference on
General Relativity and Relativistic Astrophysics. This paper is a condensed
version of gr-qc/990707
Free Space Optical Link Utilizing a Modulated Retro-Reflector Intended for Planetary Duplex Communication Links Between an Orbiter and Surface Unit
Presented are simulation and experimental results that provide duplex optical-free space communication links with minimal power and pointing requirements by using a modulated retro-reflector (MRR) for planetary communications. The design is the MRR resides on the surface of a planet or moon, where energy is scarce, while the source of the communication laser resides on an orbiter to achieve satellite-to-ground communications. Also, a simulated scenario using the Mars Reconnaissance Orbiter (MRO) is provided for real world potential results. The information sent through this communication path can range from raw scientific data to multimedia files such as videos and pictures. Bidirectional communications is established with the MRR by using a nested pulse position modulation (PPM) structure. This modulation scheme is then evaluated for its validity in a proof-of-concept experiment. Initial results indicate a promising return-link performance of at least 300 kbps in the nested arrangement
Tax Policy and Returns to Education
This paper considers how asymmetric tax treatment, where labour market earnings are taxed but household production is untaxed, aspects educational choice and labour supply. We show that taxes on labour market earnings can generate a large (non-marginal) switch to home production and the ensuing deadweight losses are large. Using a cross-country panel, we find that gender differences in labour supply responses to tax policy can explain differences in aggregate labour supply and years of education across countries.Increasing returns; tax policy; gender; labour supply; education
Comparative Monte Carlo Efficiency by Monte Carlo Analysis
We propose a modified power method for computing the subdominant eigenvalue
of a matrix or continuous operator. Here we focus on defining
simple Monte Carlo methods for its application. The methods presented use
random walkers of mixed signs to represent the subdominant eigenfuction.
Accordingly, the methods must cancel these signs properly in order to sample
this eigenfunction faithfully. We present a simple procedure to solve this sign
problem and then test our Monte Carlo methods by computing the of
various Markov chain transition matrices. We first computed for
several one and two dimensional Ising models, which have a discrete phase
space, and compared the relative efficiencies of the Metropolis and heat-bath
algorithms as a function of temperature and applied magnetic field. Next, we
computed for a model of an interacting gas trapped by a harmonic
potential, which has a mutidimensional continuous phase space, and studied the
efficiency of the Metropolis algorithm as a function of temperature and the
maximum allowable step size . Based on the criterion, we
found for the Ising models that small lattices appear to give an adequate
picture of comparative efficiency and that the heat-bath algorithm is more
efficient than the Metropolis algorithm only at low temperatures where both
algorithms are inefficient. For the harmonic trap problem, we found that the
traditional rule-of-thumb of adjusting so the Metropolis acceptance
rate is around 50% range is often sub-optimal. In general, as a function of
temperature or , for this model displayed trends defining
optimal efficiency that the acceptance ratio does not. The cases studied also
suggested that Monte Carlo simulations for a continuum model are likely more
efficient than those for a discretized version of the model.Comment: 23 pages, 8 figure
Extremality conditions for isolated and dynamical horizons
A maximally rotating Kerr black hole is said to be extremal. In this paper we
introduce the corresponding restrictions for isolated and dynamical horizons.
These reduce to the standard notions for Kerr but in general do not require the
horizon to be either stationary or rotationally symmetric. We consider physical
implications and applications of these results. In particular we introduce a
parameter e which characterizes how close a horizon is to extremality and
should be calculable in numerical simulations.Comment: 13 pages, 4 figures, added reference; v3 appendix added with proof of
result from section IIID, some discussion and references added. Version to
appear in PR
Constrictive pericarditis and rheumatoid nodules with severe aortic incompetence.
The case of a female patient presenting with constrictive rheumatoid pericarditis and aortic incompetence secondary to valvular rheumatoid nodules is described along with a review of the literature with the aim to highlight this rare cause of aortic insufficiency
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